Former Brexit secretary Dominic Raab has declared he will back Theresa May’s EU Withdrawal Agreement in a crunch Commons vote on the day the UK was scheduled to leave the European Union.

Mr Raab is the latest in a string of prominent rebels against the Prime Minister’s plan who have set aside their previous opposition in the hope of ensuring the UK leaves the EU on May 22.

But it remained unclear whether the reluctant support of hardline Brexiteers such as Mr Raab, Boris Johnson, Jacob Rees-Mogg and Iain Duncan Smith will be sufficient to secure victory for Mrs May.

A clutch of Leave-backing Conservatives including former Cabinet ministers Owen Paterson and Sir John Redwood and European Research Group vice-chairman Steve Baker made clear they will continue to oppose the agreement reached with the remaining 27 EU states in November.

And DUP Brexit spokesman Sammy Wilson said the Northern Irish party’s 10 MPs will vote against the agreement because it “betrays the wishes of the vast majority of people who voted to leave”.

Mr Wilson told MPs that passing the deal would make it “impossible to find a way of securing the kind of assurances required to make sure the United Kingdom is not broken up.”

(PA Graphics)

With Labour leader Jeremy Corbyn also ordering his party to vote against, Mrs May will need substantial numbers of Labour rebels to get her deal over the line.

In an apparent bid to woo wavering Labour MPs, Attorney General Geoffrey Cox told the Commons that the Government would have backed an amendment from the party’s Gareth Snell.

The Stoke Central MP’s amendment, giving Parliament the right to set the negotiating mandate for talks on future relations with the EU, was not selected for debate by Speaker John Bercow.

In a special Friday sitting of the Commons, MPs were debating the legally binding treaty segment of the Prime Minister’s Brexit deal, but not the accompanying Political Declaration setting out plans for the future EU/UK relationship.

Attorney General Geoffrey Cox speaks in the House of Commons (PA)

The PM’s decision to split the two parts of the package means it does not qualify as a third attempt to pass a “meaningful vote” on the Government’s Brexit deal.

Under the terms of an agreement with Brussels, a positive vote would qualify the UK for an automatic delay to May 22 of the formal date of Brexit.

Defeat would give the UK until April 12 to ask Brussels for a further extension to Brexit negotiations – which would require it to take part in May’s elections to the European Parliament – or leave the EU without a deal.

European Commission spokesman Margaritis Schinas confirmed that the agreement on its own was “necessary and sufficient” to deliver an orderly Brexit.

But he told a Brussels press conference that the deal must also be ratified by Parliament, something which Brexit legislation makes clear can only happen after the Political Declaration has been approved by MPs.

Announcing his decision to back Mrs May in Friday’s vote, Mr Raab told the House of Commons: “I cannot countenance an even longer extension and I cannot countenance holding European elections in May.”

Mr Johnson said he remained “intensely critical” of the deal, which he formerly said would make the UK a “vassal state”.

It is very painful to vote for this deal. But I hope we can now work together to remedy its defects, avoid the backstop trap and strive to deliver the Brexit people voted for

But he added: “I have come to the sad conclusion that neither this Government nor this Parliament is willing to leave with no deal. We therefore run the risk of being forced to accept an even worse version of Brexit or losing Brexit altogether.

“It is very painful to vote for this deal. But I hope we can now work together to remedy its defects, avoid the backstop trap and strive to deliver the Brexit people voted for.”

ERG members Lucy Allan and Daniel Kawczynski and former chief whip Mark Harper announced that they too would back the agreement after voting against it when it was defeated by a margin of 230 in January and 149 in March.

But Mr Paterson wrote in The Daily Telegraph that to avoid “catastrophic damage” to their party, Tory MPs must “stand up for the 17.4 million people who voted Leave, firmly resolved to deliver Brexit in full on April 12”.

In a graphic illustration of apparent despair within the Government at Mrs May’s chances, BBC Newsnight political editor Nicholas Watt said that, when he asked one Cabinet minister why the PM had called Friday’s vote, he received the reply: “F*** knows, I’m past caring, it’s like the living dead in here.”

The vote comes on the day that pro-Brexit protesters – including a group which has taken part in a two-week march from Sunderland – are set to gather outside Parliament to mark the date when the UK was due to leave the EU.

Brexit supporters who have taken part in a march from Sunderland are due to arrive at Parliament Square (Owen Humphreys/PA)

Opening debate in the Commons, Mr Cox urged MPs to take “a single, decisive step today to afford certainty to the millions of people around this country awaiting it” and guarantee a short delay to Brexit to May 22.

“What we have before us today is the legal right to extend,” he said. “No other extension is guaranteed. Every other extension would require European parliamentary elections.

“We therefore are at an important crossroads for the purposes of this nation’s future and its history and I urge all members of this House to embrace this opportunity now.”

Shadow Brexit secretary Sir Keir Starmer insisted Labour will not back the agreement in the vote expected at around 2.30pm.

He told BBC Radio 4’s Today programme: “Not only is uncoupling the Political Declaration making Brexit more blind because you don’t know where you are going, now the Prime Minister has said she is stepping down.

“So the Political Declaration, the future relationship, is going now to be determined in a Tory leadership exercise. Even if this Prime Minister gave us assurances about what she’s going to do in the future, they don’t mean anything any more.”